In a gas turbine, if the gas temperature is high during a first stage of the turbine, the efficiency for generating electric power increases. However, in order to raise the gas temperature for the first stage of the turbine, the heat-durability of the turbine blade and turbine nozzle should also be increased. As a method for raising the heat-durability of the gas turbine, film cooling by fluid on the blade surface is well known. FIG. 1 is a schematic diagram of the turbine blade of the gas turbine according to the prior art. The turbine blade consists of a main body 1 of the blade and a base 2 to attach the main body to a rotor (not shown in FIG. 1). FIG. 2 is a sectional plan of line K--K of FIG. 1. FIG. 3 is a sectional plan of the J--J line of FIG. 1. As shown in FIG. 2 and FIG. 3, three coolant passages 3a, 3b, 3c are formed in the base 2 and the main body 1. The three coolant passages are connected to a supply source of cooling fluid. The cooling fluid in the coolant passage 3a, 3b, 3c executes convective cooling through the base 2 and the main body 1. When the cooling fluid flows through the coolant passages 3a, 3b, it flows out through a plurality of outlets 8 on the leading edge 4, side wall 5, other side wall 6, tip 7. The cooling fluid in the coolant passage 3c flows out through outlets 10 on the trailing edge 9.
The outlet of coolant passage is normally formed as an ellipse. FIG. 4 is a schematic diagram of the outlet of the coolant passage on the blade surface according to the prior art. FIG. 5 is a sectional plan of line L--L of FIG. 4. As shown in FIG. 4 and FIG. 5, in the outlet 8 passing through the side wall 5 and the other side wall 6, the center line 12 of the outlet of the coolant passage is inclined in the direction of the gas stream 11 on the surface of the wall 5 (6). The cooling fluid flowing from the outlet 8 is mixed with the gas stream 11 flowing over the surface at high speed, and cools the surface by forming a film-like layer over it. As a method for setting the outlet on the surface, plural lines of the outlets 8 perpendicular to the direction of the gas stream 11 may be set as shown in FIG. 6 and FIG. 7. In order to supplement the outlets 8 on the upstream side, the outlets 8 on the downstream side, whose position is different from the position of the outlets on the upstream side, are set as shown in FIG. 8. Furthermore, in order to strengthen the film cooling effectiveness of the spread of the fluid, the diameter of the outlet 13 is gradually increased as it reaches the surface as shown in FIG. 9A and FIG. 9B. Alternatively, as shown in FIG. 10, the outlet 13 is opened at fixed intervals as it reaches the surface, thus resembling a staircase.
However, in the film cooling method in which the center line 12 of the coolant passage is inclined in the direction of the stream, the following problem occurs.
The cooling fluid flowing from the outlet 8 has a high Kinetic energy stream that crosses the direction of the gas stream flowing along the surface. Therefore, as shown in FIG. 11, a separation of the coolant as the cooling fluid flows up in a columnar shape occurs. As a result, the gas stream 11 is divided by a pillar 14 of cooling fluid flown from the outlet 8 and rolled up in the downstream area of the pillar 14. This makes it is difficult for the fluid film to cover the surface 5 (6) and therefore film cooling effectiveness reduces. Furthermore, when the outlet is shaped as shown in FIG. 9B and FIG. 10, the fluid film covers only 70% of the surface interval between neighboring outlets. In addition, the pressure of the fluid flowing from the outlet is low because of the wide outlet 13. Therefore, in the downstream area of the outlet 8 on the surface 5 (6), the gas stream 11 mixes with the cooling fluid 14, and the film cooling effectiveness is low.
On the other hand, according to the prior method shown in FIGS. 12A and 12B, the direction of the coolant passage is inclined in a direction different from the direction of the gas stream along the surface (i.e., the "lateral direction"). In this method, the fluid diffuses laterally in the direction of the gas stream. In short, the flown fluid diffuses only along the lateral area in the direction of the gas stream. The film cooling effectiveness of the fluid for downstream area is therefore low.
As another prior method shown in FIGS. 13A and 13B, the outlet is shaped as a diffusion type in addition to the specific feature of FIGS. 12A and 12B. In this method, the center line of the diffusion part is inclined in the lateral direction similar to the center line of the outlet of the coolant passage. Therefore, the film cooling effectiveness of the fluid over the downstream area is low in the same way as shoewn in FIGS. 12A and 12B.